Piezo actuator with novel contacting and production method

ABSTRACT

In the operation of piezoactuators of multilayer construction, during the polarizing or during the operation of the piezoactuator tears can arise, due to voltages, in the metallization strips that are attached to the actuator on the outside for contacting the electrode layers. It is inventively proposed to install electrically conductive contact lugs directly at the stack or on the metallization strips such that a laterally protruding region remains, so that, even given arising tears, these nevertheless extend in the protruding region of the contact lug, and the tears are electrically bridged. Electrical terminal elements are provided so as the project above or laterally at the contact lugs.

BACKGROUND OF THE INVENTION

Piezoactuators usually consist of several piezoelements arranged in astack. Each of these elements in turn consists of a piezoceramic layerwhich is provided with metal electrodes on both sides. If a voltage isapplied at these electrodes, the piezoceramic layer responds with a griddeformation which extends along a main axis to a useful longitudinalextent. Since this in turn amounts to less than 0.2% of the layerdensity along the main axis, a correspondingly higher layer thickness ofactive piezoceramic must be made available in order to achieve a desiredabsolute longitudinal extent. However, the voltage required for theactuation of the piezoelement rises with increasing layer thickness ofthe piezoceramic layer within a piezoelement. To keep this voltagewithin manageable limits, multilayer actuators are produced wherein thethickness of individual piezoelements is usually between 20 and 200 μm.

Known piezoactuators of multilayer construction thus consist of up to afew hundred individual layers altogether. For their production,piezoceramic green foils are arranged in a stack in alternation withelectrode material, and these are laminated and sintered together into amonolithic compound up to about 5 mm high. Larger actuators with largerabsolute excursion can be obtained by gluing together several suchstacks, for example. Only piezoactuators of fully monolithic multilayerconstruction have sufficiently high rigidities, particularly when largeforces must be transmitted with the piezoactuator.

For the electrical contacting of such piezoactuators of multilayerconstruction, metallization strips are attached to the exterior of thepiezoactuator, for example, or in a borehole in the middle of thesurface of the individual actuator. To connect every second electrodelayer with one of the metallization strips, for example, this must beinsulated against the intervening electrode layers. This occurs easilyin that every other electrode layer comprises a recess in the region ofthe one metallization strip, in which recess said electrode layer is notled to the metallization strip. The remaining electrode layers thencomprise the recesses in the region of the second metallization strip,in order to enable a contacting with alternating polarity. Wires for theelectrical connection are soldered to the metallization strips.

Piezoactuators whose alternating contacting occurs via recesses of theelectrode layers are piezoelectrically inactive in the contacting zone,since an electrical field cannot build up there due to the one electrodethat is missing, respectively. As a result, in the polarization as wellas in the operation of the piezoactuator, mechanical tensions build inthis piezoelectrically inactive contacting zone, which tensions can leadto tears in the inactive regions and thus at the metallization stripsparallel to the electrode layers as well. This can lead to the completesplitting of the metallization strip and produces the result that, givenpunctate voltage supply to the metallization strips from outside, a partof the piezoactuator becomes dependent on the power supply and thusbecomes inactive. The number of tears depends on the total height of theactuator and on the stability of the boundary surface between the innerelectrode and the piezoceramic and can rise in continuous operationgiven alternating load conditions. Since, in the dynamic operation, adynamic changing of the tears, or respectively, the tear openings alsoderives, the metallization strips are thereby further damaged during theoperation of the actuator.

SUMMARY OF THE INVENTION

It is the object of the present invention to propose a ceramic actuator,along with a method for production, which has electrical contacting thatcan be handled securely and easily and which demonstrates an increasedstability with respect to a tear formation.

The inventive actuator can have a conventional and preferably amonolithic construction. Piezoelectric ceramic layers and electrodelayers are arranged upon one another in alternation in the manner of astack and are preferably sintered together. For alternating contactingof the electrode layers, at least two electrically conductive contactlugs are inventively provided at the stack on the outside. These areconnected to the electrode layers via an edge and extend over the entireheight of the electrically active region of the stack. To the side ofthe connected edge, they comprise a protruding region, and in the regionof the outer edge that is averted from the stack, they comprise anelectrical terminal element which projects laterally or which projectsbeyond the stack in height.

With the contact lug, it is possible to bridge, in an electricallyconductive manner, tears in the metallizations that may arise in theoperation of the actuator. If the projecting region is selected so as tobe sufficiently wide, then the tears end inside the contact lug, orrespectively, inside the projecting region. All individual elements ofthe actuator thus remain electrically functional, even if tears arise atthe metallizations. The inventive actuator thus does not demonstrate anypower losses whatsoever in operation.

The terminal element at the outer edge enables the simple connection ofthe contact lug to an external current or voltage supply. It protrudesbeyond the contact lug laterally or beyond the stack in height and isthus still easily accessible given the installation of the stack in ahousing and enables a simple current connection.

In the simplest embodiment, the terminal element is produced from thematerial of the contact lug, or respectively, is an integral componentof the contact lug. This comprises at least one electrically conductivelayer. It preferably consists of a compound material with at least oneplastic film and at least one metallic film or layer, however. This typeof compound material comprises a high flexibility and a high elasticityand resistance to tearing, at the same time. Geometrically, the terminalelement represents an extension of the outer edge, which is averted fromthe stack, of the contact lug in an upward direction, or an extension ofthe upper edge to the outside, or respectively, to the side. Besides thethus simplified connection, the terminal element also serves for easierhandling of the contact lug, or respectively, of the actuator that isprovided with the contact lug, particularly during the installation intoa housing. The terminal elements can serve as a guide here.

The terminal element is preferably constructed such that it representsan additional mechanical reinforcement of the outer edge of the contactlug. In an advantageous development, the terminal element is constructedas a metallic terminal pin. This can be soldered or otherwiseelectrically conductively fastened on the contact lug, or respectively,on its metallic layer. The terminal pin can extend over the entire outeredge or can be connected only to a part of the edge. The guidance, orrespectively, handling of the contact lug that is fastened at the stackis simplified with a mechanically reinforcing terminal element such asthis.

The terminal element, which is constructed as a terminal pin, can alsobe part of an electrical plug connection. This enables an extremelysimple connection to an electrical voltage source.

The actuator preferably comprises a pressure plate at a face of thestack. This has openings through which the terminal elements areinserted or guided. In mechanically reinforced terminal elements, theopenings are designed such that a guidance and retaining of the terminalelements is guaranteed.

The pressure plate preferably has a recess for fixing the stack. Thisenables a secure installation of the stack, including the contact lug,in an actuator housing, whereby the stack is securely oriented andcentered.

In the pressure plate, other elements of the actuator can be integrated,such as force sensors, temperature sensors, other sensors (e.g. Hallsensors) or a second actuator as an adjusting element for rigidity,temperature expansion compensation and zeroizing.

For the mechanical protection of the stack in the actuator, this has aplastic envelope. For this purpose, the stack is preferably cast into aplastic sheath or coated with a plastic. Elastomers on a silicon resinbase are particularly suitable for this. The installation into theplastic sheath can occur with the aid of the pressure plate, whereby asimple guidance and fixing of the stack, contact lug and terminalelement is achieved.

In the longitudinal direction of the actuator, the other previouslymentioned elements which complete the actuator can be simultaneouslyinstalled in the envelope, in the casting or coating with plastic aslong as they are not installed elsewhere, e.g. in the pressure plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures of which like referencenumerals identify like elements, and in which:

FIG. 1 shows a schematic view of an actuator with contact lug andterminal element, in perspective view.

FIGS. 2 to 4 show various contact lugs in a schematic view.

FIGS. 5 and 6 show schematic cross-sections through inventive actuatorswith pressure plates.

FIG. 7 shows a schematic cross-section of an actuator with a plasticsheath.

FIG. 8 shows a schematic perspective view of the installation of a stackinto a plastic sheath.

FIG. 9 shows an actuator which is assembled from several stacks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a section of an inventive actuator, whereby, for thesake of clarity, only one contact lug KF is illustrated. The core of theactuator is the stack S, which is assembled from electrode layers E1, E2and ceramic layers KS in alternating fashion. The actuator, which isillustrated here with a square floor area, is provided withmetallization strips MS at opposite corners, which strips arerespectively connected to every other electrode, due to the geometricdesign of the electrodes E, so that a parallel connection of allindividual actuator elements is possible. A contact lug KF1 is fastenedat the first metallization strip MS1 over its entire height with oneedge such that a part, or respectively, the remainder of the contact lugprojects laterally at the stack. In the region of the outer edge, whichis averted from the stack, a terminal element AE is arranged, whichprojects beyond the stack in height or laterally. While the width of theprotruding region a is sufficiently selected such that tears in themetallization and particularly in the metallization strips MS whicharise during the operation or the poling of the actuator end within thecontact lug KF, the extent to which the terminal element AE projectsbeyond the contact lug KF, or respectively, the stack S in height orlaterally depends on the further construction of the actuator, itshousing or the other electrical terminals.

FIG. 2 shows the simplest development of the invention, wherein thecontact lug and terminal element are prefabricated from an electricallyconductive film, such as a copper-lined plastic film. In the region ofits inner edge, the contact lug has a narrow solder layer LS with theaid of which a simple soldering of the contact film to the metallizationstrip MS is possible. Alternatively, the contact lug can also befastened directly at the stack, or respectively, at the electrode layerswithout metallization strips with the aid of the solder layer.

A preferred and gentle method of fastening is laser beam soldering,which only generates a minimal thermal load on the heat-sensitivemultilayer structure.

FIG. 3 shows another development of the contact lug KF, which in turn isconstructed as a film with at least one electrically conductive layer.The terminal element AE is constructed here as a metallic terminal pinwhich is electrically conductively connected to the contact lug KF inthe region of an outer edge and is soldered on, for example. Theterminal pin comprises a round cross-section, for example.

FIG. 4 depicts an additional possible development of the contact lug KFin which the terminal element AE represents a lateral extension of thecontact lug. As is illustrated in the figure, it can be connected to ametallic terminal pin, thus forming a combined terminal element.

In all cases, the contact lug KF is fully fabricated prior to itsfastening at the metallizations or directly at the stack; i.e., it isprovided with terminal elements AE and with the solder layer LS, aswarranted.

During the soldering or otherwise electrically conductive fastening ofthe contact lug at the metallizations, these are electrically shorted,in order to prevent a damaging of the (for instance) piezoelectricactuator via the pyroelectrical effect. Using the same measure, damageis always avoided in later processing steps when thermal loading of thestack can be expected.

FIG. 5 depicts a submodule of the actuator, consisting of the stack S,which is provided with the contact lugs KF, and a pressure plate DP,which comprises bushings for the terminal elements AE. Particularly inmechanically reinforced terminal elements, such as the metallic terminalpins, the bushings DF serve to lead and restrain both the terminalelements and the connected contact lug KF. The pressure plate DP isarranged over a face of the stack S and comprises a depression 50 to 100μm deep which is adapted to the face for purposes of fixing the stack.With this submodule, the further processing, such as the installationinto an actuator housing, is simplified, since the orienting andcentering of the actuator, which is necessary for an optimaltransmission of forces, is simplified by the fixing of stack S, contactlug KF and terminal elements AE in the pressure plate DP.

FIG. 6 shows a modified submodule with contact lugs KF and terminalelements AE which are fashioned according to FIG. 4. As in FIG. 5, herealso the terminal elements are fixed in lead-throughs DF, and the stackis fixed in a recess of the pressure plate. In this development it ispossible to first secure the contact lugs at the metallization strips ofthe stack S without terminal elements AE, and to connect the pressureplate DP fixedly to the terminal elements AE. In this embodiment, anelectrically conductive connection of the terminal elements to thecontact lugs is performed after the joining of pressure plate and stack,by soldering, for example. This embodiment has the advantage that thepressure plate and terminal elements form a secure unit which cansimultaneously serve as a plug contact for connection to acurrent/voltage source. The sealing of the lead-throughs can be donewithout regard for a mechanical of thermal loading or the actuator andcan thus be constructed particularly tight.

FIG. 7: The submodule of pressure plate, terminal elements, contact lugsand stack is provided with an envelope for electrical passivation andfor mechanical protection. In the exemplifying embodiment, a sheath H isprovided for this purpose, which consists of plastic or metal, forexample, is adapted to the size of the submodule, and closes with thepressure plate in a sealed manner. In this sheath H, the submodule isinserted and is subsequently cast or coated with plastic K, e.g. asilicon elastomer, via an opening which remains open. Not only are allelectrically active surfaces of the stack and of the contact lugsinsulated electrically and from environmental influences, but amechanically secure connection between sheath H, pressure plate DP andstack S is also produced. Given a plastic sheath H, the bottom face ofthe stack thus remains free, since a better force transmission ispossible with the preferably ceramic face of the stack than with thesheath.

Prior to the casting or injection of the submodule into a sheath, it ispossible to additionally passivate the electrically active surfaces ofthe ceramic multilayer stack S, particularly by the deposition of anelectrically insulating, sufficiently elastic plastic compound, such asa silicon elastomer. The passivation is simplified by the retaining ofthe contact lug via the terminal elements that are fixed in the pressureplate. In addition, the passivation prevents a short between a contactlug and an electrically active surface region of the stack due tomechanical forces which arise in the coating or casting and which act onthe contact lugs KF, which short could lead to a malfunctioning of theactuator.

If the deposition of the passivation occurs prior to the production of asubmodule from the stack and pressure plate, then the flexible contactlugs KF can be fixed elsewhere via the terminal elements AE, so that thepassivation, e.g. by pencil deposition, is simplified. To this end, theterminal elements and potentially the stack as well can be fixed in anauxiliary holding device. The passivation of the stack surfaces prior tothe assembling of said submodule has the advantage that, prior tofurther processing, the contact lugs KF can now be arranged nearer thestack and can also be placed adjacently, for example. This enables aspace-efficient arrangement in a sheath H. A stack with contact lugsthat are adjacent at the side surfaces is mechanically more stable andis also protected against mechanical damage during the installation inthe sheath or in an actuator housing. In a development of the invention,it is possible to press the contact lugs to the side faces of the stackand to fix them there, for example by means of an elastic plastic ringwhich is pushed over the stack along with the adjacent contact lugs andterminal elements and which presses the contact lugs and terminalelements tightly to the stack.

FIG. 8 illustrates another development of the invention with which asecure installation of the stack together with the contact lug andterminal elements in a plastic sheath H is possible. The plastic sheathis not constructed as an empty hollow cylinder but rather alreadycomprises an inner profile which is adapted to the, geometric shape ofthe stack, contact lug and terminal elements. Subsequent to theinsertion of the stack into the preformed inner profile of the sheath H(see arrow b), the stack as well as the contact lugs and terminalelements are sufficiently fixed. In the subsequent filling of theremaining interspaces to the inner profile with plastic, a damaging ofthe contact lugs, or respectively, an undesirable short between contactlugs and potentially exposed electrically active surfaces of the stack,is avoided. In this embodiment also, the terminal elements AE as well asthe stack S can be fixed in a holding device such as a pressure plate DP(not illustrated in FIG. 8) in order to enable a simple insertion of thestack into the sheath H and also the enable a one-sided hermeticallytight sealing of the stack in the sheath.

FIG. 9 depicts another development of the invention, which can becombined with all the illustrations previously described. This comprisesa multilayer construction consisting of two stacks S, S′ that arearranged one on top of the other, the stacks S and S′ being connected bymeans of common contact lugs KF and thus fixed relative to one another.Mechanically reinforced terminal elements AE such as metallic terminalpins additionally stabilize the arrangement. In this way, it is possibleto achieve the necessary total height of the multilayer construction andthus of potential excursion of the actuator, despite the low stacks,which are technologically easier to produce, and also to achieve asimple and elegant connection of the two substacks S, S′. Auxiliaryholding can also serve here for early centering of a plurality of stacksover one another, it being possible for these to be already arranged ata pressure plate and oriented. This simplifies the laser beam solderingof the common contact lugs KF to the stacks S and S′ and alsofacilitates the centering of the stack in the injection molding.

The invention is not limited to the particular details of the method andapparatus depicted and other modifications and applications arecontemplated. Certain other changes may be made in the above describedmethod and apparatus without departing from the true spirit and scope ofthe invention herein involved. It is intended, therefore, that thesubject matter in the above depiction shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A ceramic actuator, comprising: a multilayerconstruction, having at least one stack of alternating electrode layersand ceramic layers; at least two strip-shaped electrically conductivecontact lugs which are attached to the stack laterally; each contact lugbeing connected to the electrode layers in an electrically conductivemanner via an edge, and each contact lug having a protruding region to aside of said edge; an electrical terminal element on each contact lug ina region of an outer edge of the contact lug that is averted from thestack, said element being one of projecting beyond the contact luglaterally or beyond the stack in height; and the terminal element has ametallic terminal pin which mechanically reinforces the outer edge ofthe contact lug and which extends along the outer edge over an entireheight of the contact lug.
 2. The ceramic actuator according to claim 1,wherein the terminal element has a strip-shaped extension of the contactlug which is connected to the metallic terminal pin.
 3. The ceramicactuator according to claim 2, wherein a pressure plate is arranged overa face of the stack, through which plate the terminal pins are insertedand fixed thereto.
 4. The ceramic actuator according to claim 1, whereinthe terminal pins are each constructed as a plug contact.
 5. The ceramicactuator according to claim 3, wherein there is a recess provided in thepressure plate, in which recess the stack is fixed.
 6. The ceramicactuator according to claim 1, wherein the stack and the contact lugsare coated with plastic on all sides or are cast in a compact plasticenvelope, and wherein the electrical terminal elements are led out ofthe plastic envelope at a face side thereof.
 7. The ceramic actuatoraccording to claim 6, wherein the plastic envelope is constructed from asilicon elastomer.
 8. The ceramic actuator according to claim 1, whereinseveral stacks are arranged one upon the other and are connected tocommon terminal lugs.